Method of Agglomeration

ABSTRACT

A method of agglomerating a nickel laterite ore comprising the steps of: i) feeding a nickel laterite ore material to be agglomerated into an agglomeration circuit; and ii) adding a sulfuric acid containing agglomerating solution to the ore material in the agglomeration circuit to provide an agglomerated ore material, wherein the acid concentration of the agglomerating solution is between 100 to 400 g/L.

FIELD OF THE INVENTION

The present invention relates to a method of agglomeration of nickellaterite ores, particularly to the agglomeration of ores for the purposeof heap leaching.

BACKGROUND ART

The following discussion of the background to the invention is intendedto facilitate an understanding of the invention. However, it should beappreciated that the discussion is not an acknowledgement or admissionthat any of the material referred to was published, known or part of thecommon general knowledge as at the priority date of the application.

Heap leaching is one method for economically extracting metals fromlow-grade ores. Generally, it involves piling raw ore into heaps andintroducing a leaching solution over the top of the heap to percolatedown through the heap. The leaching solution extracts the desired metalvalues from the ore and is collected as a pregnant leach solution (PLS),for example, in perforated drainage pipes located beneath the heap. ThePLS is then transported to a processing plant where the desired metalvalues are separated from the impurities in the PLS and recovered.

An agglomeration process can be used to bind finer particles in the oreto form larger particles and thereby improve an ore's leachingcharacteristics. In heap leaching, larger particle sizes facilitatespercolation of leach solutions through the heaped ore material andensures that the target metal values contained in the fine materials arecontacted by the leaching solution.

U.S. Pat. No. 6,312,500 in the name of BHP Minerals International Incdescribes one method of agglomerating laterite ores. In this patent, theore is initially crushed, when necessary, to a particle size range ofless than one inch (2.54 cm). The particles are then agglomerated bymixing the particles with a concentrated sulfuric acid in a rotary diskor drum agglomerator. The agglomerated particles are generally cured inopen air, undisturbed for a time period long enough so that the sulfuricacid has enough chemical reaction time to attack the nickel containingminerals, partially dissolve some of the ore, remove some of thecontained water (by air evaporation) and allow basic sulfates and/orsilicates, or silica gel to be precipitated and form into a bindingagent to keep the remaining ore particles together in a pellet form. Thecuring time can range from one hour to several days.

The use of strong or concentrated acid in an agglomeration process isadvantageous due to the generation of heat from the exothermic reactionof the acid and the small amount of moisture present in the ore. Thiscan result in better break-down of the ore minerals and exposing thetarget metal values. However, one of the main problems with usingconcentrated acid solutions for agglomeration is that the resultingagglomerates can become friable, resulting in blockages in the heap onceagglomerates begin to break into smaller particles. The use ofconcentrated acid can also promote dissolution of unwanted impurities.

Greek patent GR1001555 in the name of Agatzini teaches a heap leachingprocess in which low to very low grade nickel and cobalt oxide ores canbe agglomerated by wetting the ore with either water or leach solution(lixiviant) which preferably contains between 1 N to 2 N (49 to 98 g/L)sulfuric acid.

International patent publication WO2007143779 in the name of MurrinMurrin Operations Pty Ltd relates to a method for agglomeration of aNickel Laterite ore where the moisture content of an ore material to beagglomerated is adjusted to a predetermined level, and that ore materialis agglomerated using an acid containing agglomerating solution havingan acid concentration that does not exceed about 100 g/L. The use of adilute acid solution for agglomeration is taught as providing a suitableagglomerated ore material (“agglomerate”) that does not sacrifice thestructural integrity of the agglomerates. Heap leachate percolationrates and extraction levels are also taught as being improved.

The applicant's have surprisingly found that agglomeration of nickellaterite ores is optimised using a selected range of conditions thatfall outside the conditions covered by GR1001555, U.S. Pat. No.6,312,500 and WO2007143779.

Throughout the specification, the terms “ore” and “ore material” areunderstood to include nickel laterite in the form of any one of ore, orereject material, concentrate, waste rock or mill scats material.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method ofagglomerating a nickel laterite ore comprising the steps of:

-   -   i) feeding a nickel laterite ore material to be agglomerated        into an agglomeration circuit; and    -   ii) adding a sulfuric acid containing agglomerating solution to        the ore material in the agglomeration circuit to provide an        agglomerated ore material, wherein the sulfuric acid        concentration of the agglomerating solution is between 100 to        400 g/L.

The applicant has found that despite the teaching of prior patentpublications, nickel laterite ore is optimally agglomerated using anagglomerating solution having a sulfuric acid concentration of between100 to 400 g/L and preferably between 140 to 280 g/L. Optimalagglomeration of nickel laterite ore improves the permeability of theore heap, and enables the agglomerated ore to be piled in higher heapsas compared to prior agglomerated ore, which in turn allows a heapleaching process to achieve more efficient recovery rates, increaseoutput rates and reduce costs.

The agglomerating method of the present invention can be used for thelimonitic portion of the ore or the saprolitic portion which may beselected to be leached separately, or for a whole of ore mix

The agglomerating solution includes sulphuric acid content capable ofagglomerating smaller particles of nickel laterite ore into largerparticles. The agglomerating solution may also contain water in the formof water, underground brine, saline solution, hypersaline solution, seawater, recycle solution or a mixture thereof. In some embodiments, theagglomerating solution may comprise a pregnant leach solution (PLS) froma downstream heap leach circuit.

The amount of sulfuric acid added to the agglomeration step per tonne ofore can vary depending on the actual composition of the nickel lateriteore to be agglomerated, the moisture content of the ore, the acid usedand other factors. Typically, agglomeration is achieved by addingbetween 20 and 50 kg of acid per dry tonne of ore material. In apreferred embodiment, agglomeration is achieved by adding about 35 kg ofacid per dry tonne of ore.

In some embodiments, it may be advantageous to reduce the size of theore before the agglomeration step. In these embodiments, the process ofthe present invention further includes a step prior to the ore feedingstep of reducing the size of the nickel laterite ore material to beagglomerated to less than 1 inch. The ore can be reduced in size usingany suitable size reduction equipment including ball mills, crushers,hammer mills or the like.

The moisture content of the ore has been found to affect theagglomerating step of the present invention. It is therefore preferablefor the ore to be pre-treated to control the moisture content of the orematerial to be agglomerated. Accordingly, in some embodiments, theprocess of the present invention further includes a step prior to theore feeding step of adjusting the moisture content of an ore material tobe agglomerated to a predetermined level. For example, in someembodiments, the initial (natural_moisture content of the ore materialprior to agglomeration can be between 1% and 40% wt/wt. This initialmoisture content of the ore material can be adjusted prior toagglomeration to a selected moisture content between 1% and 40% wt/wt.In other embodiments, the moisture content of the ore material isadjusted prior to agglomeration to be between 5% and 30% wt/wt. In yetothers, to between 15% and 25% wt/wt. After agglomeration the finalmoisture content of the ore material can be adjusted to be between 5%and 40% wt/wt with the addition of or without the addition of aqueoussolution. However, it is to be understood that other ranges may besuitable for an individual ore. This level will vary for individualores, depending on the source and mineralogy, and will be determined bytest work for each case.

This step can be done either after, before or during the size reductionstep if such a step is undertaken.

The agglomerated ore is preferably cured for an amount of time that mayvary from an hour to several days, depending on the completion of thechemical bonding process that takes place within the resultantparticles. This will vary for each individual ore, depending on the oresource and mineralogy. In most cases, the optimum curing time will needto be established experimentally for each individual ore. In theseembodiments, the process of the present invention further comprises astep, after the acid addition step of curing the agglomerated ore for apredetermined curing time. The curing time of the agglomerated orematerial can be anywhere between 1 hour to several days.

The agglomeration circuit of the agglomeration step can include anysuitable agglomeration equipment, for example one or more drum or rotarydisc agglomerator.

DETAILED DESCRIPTION

The present invention will now be described with reference to particularpreferred embodiments of the present invention.

In accordance with the present invention, a method of agglomeratingnickel laterite ores is provided for improving the heap leachingcharacteristics of that laterite ore.

In a typical nickel laterite heap leaching process, the particle size ofthe nickel laterite ore is first reduced for example by crushing in aball mill, where necessary, to an average particle size of less than2.54 cm and, preferably between about 6 mesh (3350 micron) and about 2cm

In some embodiments, the nickel laterite ore is subjected to a moistureadjustment step in which the initial (natural) moisture content of theore is adjusted to a predetermined level by air drying the ore in aheap, or by adding water to the ore, as required before agglomeration.For example, in wet conditions (rain or similar conditions), atoo-wetted laterite may be sun-dried to decrease the initial moisture.The final moisture content of the ore is determined with bench or smallcontinuous operation to determine how much extra water/solution inaddition to concentrated acid can be added in ore to create qualifiedpellets for good permeability.

The crushed particles are then fed into an agglomeration stage to beagglomerated. In this stage, the particles are mixed with anagglomerating solution, preferably a solution containing between about140 and 280 g/L of sulphuric acid, to agglomerate the ore particles. Theagglomeration step may be carried out in a rotary disk, drum, or othersuitable apparatus known in the art. The sulfuric acid may be mixed witha source of water, including saline, underground brine, fresh water, seawater, hypersaline solution or recycle solution. The quantity ofsulfuric acid used is generally that necessary to attack the acidconsuming mineral materials in the ore (generally the readily availableMgO from magnesium silicates that are present in the ore). It has beenfound that about 35 kg of sulfuric acid per ton of ore (dry base).However, the quantity can range from about 20 to about 50 kg sulfuricacid per ton of ore (dry base). The resulting agglomerated particles(pellets) have an average particle size between about 6 mesh (3350micron) and about 2.54 cm.

The agglomerated pellets are generally cured for an amount of time thatmay vary from an hour to several days, depending on the completion ofthe chemical bonding process that takes place. Preferably, the curingtime ranges from about 4 hours to 10 hours. During curing, theagglomerated ore particles (usually in the form of pellets) are storedin open air, undisturbed for a time period long enough so that thesulfuric acid will have enough chemical reaction time to partiallydissolve some of the ore and to remove some of the contained water byair evaporation such that basic sulfates and/or silicates, or silica gelis precipitated and formed into a binding agent to keep the remainingore particles together in pellet form. The chemical bonding processconsists of resolubilisation of precipitated silica gel and thedissolution of iron and magnesium from the ore with the strong acid usedfor pelletization. During the curing process, excess water evaporatesand some of the water is taken as crystal water and causes are-precipitation of magnesium-iron-silica gel “glue” that acts as achemical bonding agent between the solid particles making solid pelletsthat have a definite physical strength. The curing time will be afunction of the quantity of excess, or free, water that needs to beremoved from the “glue” phase.

The agglomerated pellets are then formed into a heap having a base and atop. A leach solution is applied to the top of the heap and allowed topercolate downward through the heap. The PLS is collected at the bottomof the heap and may be recycled, collected for nickel (and/or cobalt)recovery, or a combination of both. The leach solution is preferablysulfuric acid. The leach solution may be a recycled solution from whichpay metal values have been recovered. Preferably, the pelletizedagglomerated ore is formed into two, or a series of, heaps and leachedin a counter-current process.

In the practice of the present invention, acid in the range of about 20to about 50 kgs of sulfuric acid per ton of ore is all that is requiredto produce a pellet with the desired physical characteristics towithstand handling and pile loads that are incurred from heaping theore. Overall, optimum agglomerating acid dose for agglomeration based ona Knappes permeability test is around 35 kgs of sulfuric acid per ton ofore using an agglomerating solution of 140 to 280 g/L sulfuric acid.

Where the terms “comprise”, “comprises”, “comprised” or “comprising” areused in this specification (including the claims) they are to beinterpreted as specifying the presence of the stated features, integers,steps or components, but not precluding the presence of one or moreother feature, integer, step, component or group thereof.

1. A method of agglomerating a nickel laterite ore comprising the stepsof: i) feeding a nickel laterite ore material to be agglomerated into anagglomeration circuit; and ii) adding a sulfuric acid containingagglomerating solution to the ore material in the agglomeration circuitto provide an agglomerated ore material, wherein the acid concentrationof the agglomerating solution is between 100 to 400 g/L.
 2. A methodaccording to claim 1, wherein the acid concentration of theagglomerating solution is between 140 to 280 g/L.
 3. A method accordingto claim 1 wherein agglomerating solution is added at between 20 and 50kg of acid per tonne of ore material.
 4. A method according to claim 3wherein agglomerating solution is added at about 35 kg of acid per tonneof ore.
 5. A method according to claim 1 further including a step of:adjusting the moisture content of an ore material to be agglomerated toa predetermined level, prior to feeding the ore into the agglomerationcircuit.
 6. A method according to claim 5, wherein the moisture contentof the ore material is adjusted prior to agglomeration to be between 5%and 30% wt/wt.
 7. A method according to claim 1 further including a stepof: reducing the size of the nickel laterite ore material to beagglomerated to less than 1 inch, prior to feeding the ore into theagglomeration circuit.
 8. A method according to claim 1 furthercomprising the step of curing the agglomerated ore for a predeterminedcuring time.
 9. A method according to claim 8, wherein the curing timeof the agglomerated ore material is between 1 hour to 10 days.
 10. Amethod according to claim 8 wherein the curing time of the agglomeratedore material falls within the range of about 1 hour to 10 hours.
 11. Amethod according to claim 1 wherein the agglomeration circuit includesone or more drum or rotary disc agglomerators.
 12. A method according toclaim 1 wherein the agglomerating solution comprises a pregnant leachsolution from a downstream heap leach circuit.